How do you determine the characteristic strength of concrete?

Strength of concrete is tested by destructive testing of concrete test core samples that are taken at the time the concrete is poured. The test cores are about 7" diameter x 16" tall plastic containers. A number of samples are taken. One will be tested at 7 days, another at 28 days. A test report is provided to the firm responsible for paying the bill. The testing method is: place the core sample (removed from the plastic container), in the test machine; the machine will compress the sample until it breaks. A gauge measures the force applied and at what point it breaks.

Concrete usually breaks at 30-40% higher than the minimum specified, because the concrete company has to guarantee the concrete strength. Concrete strength is specified as: 2500 pound, 3000 pound, 4,000 pound, 6000 pound concrete. Sidewalks and non load bearing loads use the lighter weight concrete, bridges and buildings carrying loads will use a higher strength concrete. The strength is adjusted by adding or subtracting the percentages of the raw materials in the mix; limestone, water, sand, and mortar mix.